New insights into the involvement of residue D1-V185 in Photosystem II function inSynechocystis6803 andThermosynechococcus vestitus.

The effects of D1-V185T and D1-V185N mutations in Photosystem II (PSII) from Thermosynechococcus vestitus (formerly T. elongatus ) and Synechocystis 6803, respectively, were studied using both EPR and optical kinetics. EPR spectroscopy reveals the presence of a S 2 state in a high-spin configuration (S 2 HS ) and in a low-spin configuration (S 2 LS ) in both mutants. In contrast to the (S 2 HS in the wild type, the (S 2 HS state in the D1-V185T mutant does not progress to the S 3 state at 198 K. This inability is likely due to alterations in the protonation state and hydrogen-bonding network around the Mn 4 CaO 5 cluster. Optical studies show that these mutations significantly affect proton release during the S 3 -to-S 0 transition. While the initial fast proton release associated with Tyr Z ● formation remains unaffected within the resolution of our measurements, the second, and slower, proton release is delayed, suggesting that the mutations disrupt the hydrogen-bonding interactions necessary for efficient deprotonation of substrate water (O6). This disruption in proton transfer also correlates with slower water exchange in the S 3 state, likely due to non-native hydrogen bonds introduced by the threonine or asparagine side chains at position 185. These findings point to a critical role of D1-V185 in regulating both proton transfer dynamics and water binding, underscoring a complex interplay between structural and functional changes in PSII.